Sergio M. Alcocer

Backbone model for confined masonry walls for performance-based seismic design

In this study, a performance-based model is proposed, capable of simulating seismic behavior of typical confined masonry (CM) walls whose response is governed by shear deformations. This model is developed on the basis of both monotonic and reversed-cyclic experiments assembled in an extensive database, and derived through an iterative linear regression analysis. Owing to the limited data available and inconsistencies in observed behavior in some tests, only specimens with two tie columns, one on either edge of the wall; multiple longitudinal rebar per confining element; no bed joint reinforcement; no openings within the confined panel; and a height-to-length ratio that varies from 0.7 to 1.2, are considered for the purpose of model development. The effect of openings on strength characteristics, the capability of existing models to predict seismic behavior of CM walls, and the limitations of the proposed equations are discussed in detail. The accuracy of the model is also verified for CM walls with different characteristics. The proposed model simulates reasonably well the seismic behavior of CM walls whose properties conform to the assumptions of the model and that correspond to typical CM walls.

Backbone Model for Performance-Based Seismic Design of RC Walls for Low-Rise Housing

The walls of modern low-rise economic housing in several Latin American countries are typically thin and have a low concrete strength, web steel ratios that are smaller than the minimum prescribed by current codes, and web shear reinforcement made of welded-wire mesh. In light of these particular wall characteristics, research was aimed at developing a performance-based backbone model capable of predicting the seismic behavior of reinforced concrete (RC) walls for one- and two-story housing. The selected tri-linear model is associated with three limit states: diagonal cracking, peak shear strength, and ultimate deformation capacity. The model was developed on the basis of the observed response of 39 quasi-static and shake table experiments. Iterative nonlinear regression analyses were performed for deriving the semi-empirical equations in this study. The paper also discusses the adequacy of some existing models to predict the seismic behavior of RC walls and the limitations of the proposed equations.

An Experimental Study of Confined Masonry Walls with Varying Aspect Ratios

Results from an experimental series of seven full-scale confined masonry walls with height-to-length aspect ratios (H/L) from 0.3 up to 2.2 are summarized. Results show that neither the level of axial stress nor the aspect ratio had a significant effect on lateral stiffness. Inelastic behavior of the walls, characterized by normalized stiffness degradation with ductility demand, can be estimated with good accuracy with a bilinear function for a ductility demand up to 4.5. A substantial increase in normalized shear strength was observed for walls with decreasing aspect ratio. A correction factor to the nominal cracking strength was deduced based on differences of the flexural deformations for squat and square walls. The factor was then compared to the experimental normalized strength with good agreement. A new expression for inclined cracking shear that can be used for a wide range of wall aspect ratios is proposed.

Seismic Perf ormance Evaluation of Rehabilitated Reinforced Concrete Columns through Jacketing

Rehabilitation techniques for seismically deficient reinforced concrete elements were developed more than 20 years ago. At that time, lateral-load response of rehabilitated elements was deemed satisfactory if significant strength degradation did not occur at a target displacement demand (typically large). Large economic losses in recent earthquakes have highlighted the importance of evaluating element response at other performance levels, such as those associated with smaller displacements. In order to accurately estimate the global response of rehabilitated reinforced concrete buildings, a thorough understanding on the response of individual rehabilitated components must be developed. ASCE/SEI Standard 41-06 – Seismic Rehabilitation of Existing Buildings, provides engineers with a much needed resource to incorporate performance-based concepts in seismic rehabilitation projects. Within ASCE/SEI 41-06, guidelines to develop backbone curves for modeling typical reinforced concrete elements, and acceptance criteria of these elements based on their calculated deformation are provided. The document, however, does not explicitly include information that can be used for elements that have been rehabilitated by methods such as jacketing. A need to examine the performance of these elements using a multi-level performance perspective is therefore warranted. In this paper, a comparison of the laboratory performance of jacketed reinforced concrete columns with modeling parameters in ASCE/SEI 41-06 is presented. This information is part of ongoing activities taking place within ACI Committee 369 – Seismic Repair and Rehabilitation, that focus on an evaluation of existing guidelines for the seismic rehabilitation of concrete buildings. Existing laboratory tests found in the literature are used to compare the plastic deformation capacity of jacketed reinforced concrete columns with the corresponding modeling parameters in ASCE 41-06. The three primary types of jacketing materials that have been used to date (concrete, steel, and fiber-reinforced polymers) are included in the study. Particular attention is paid on whether plastic drifts in jacketed columns compare favorably with modeling parameters in ASCE/SEI 41-06.

RECENT EXPERIMENTAL EVIDENCE ON THE SEISMIC PERFORMANCE OF REHABILITATION TECHNIQUES IN MEXICO

Observed performance in the laboratory of masonry and structural concrete walls and columns rehabilitated by jacketing is reported. Improved behavior is discussed in terms of strength, stiffness, and energy dissipation and deformation capacity. In general, wall and column jacketing, when properly detailed, provide a reliable rehabilitation technique suitable for rehabilitation of highly vulnerably structures.

Evolution of codes for structural design in Mexico

Purpose – The purpose of this paper is to provide an overview of structural design code development in Mexico, as an example of how code evolution has taken place in many countries, and to discuss the challenges faced in improving code compliance and reducing vulnerability.Design/methodology/approach – All building codes for Mexico City (1920‐2004) and current codes in Mexico were analyzed and compared in terms of their scope and contents. Emphasis is placed on understanding how design requirements, professional liabilities and overall performance requirements have evolved.Findings – Structural codes have become more refined and complex, including at each revision the current state‐of‐the‐art knowledge. However, code compliance and code misinterpretation are prevalent, mainly because users are not familiar with the concepts and technologies involved. Also, it is stated that government authorities lack the technical capacity to identify code misinterpretations. Finally, it is stated that, for non‐engineere...

Propiedades mecánicas del concreto para viviendas de bajo costo

La resistencia a cortante de muros de concreto y los nuevos materiales y tecnicas de construccion han posicionado a la vivienda industrializada de concreto como una opcion eficiente para proporcionar seguridad ante eventos sismicos, para incentivar la conservacion del medio ambiente y para promover la reduccion de los costos de construccion, operacion y mantenimiento. Con el proposito de desarrollar ayudas de diseno que promuevan la utilizacion de diferentes tipos de concreto, se llevo a cabo un estudio experimental para caracterizar las propiedades mecanicas de los concretos de peso normal, peso ligero y autocompactable. El programa experimental incluyo el ensayo de 603 especimenes en forma de cilindros y vigas. En el estudio se determinaron las propiedades mecanicas de los tres tipos de concreto sometidos a esfuerzos de compresion, tension y flexion. A partir de las tendencias de los resultados experimentales, se proponen correlaciones numericas para estimar las propiedades mecanicas basicas de los concretos, tales como modulo de elasticidad, resistencia a tension indirecta y resistencia a flexion. Adicionalmente, en el estudio se proponen recomendaciones respecto a la resistencia minima a compresion del concreto para vivienda y a la edad de descimbrado de los muros. Las recomendaciones de este estudio se podrian implantar facilmente en un reglamento para construccion de vivienda de baja altura y de bajo costo.

Finite element/percolation theory modelling of the micromechanical behavior of clayey soils

A hybrid model for soils, which combines percolation theory and finite element method is presented. The internal soil structure is modelled via the finite element method, and percolation networks are used for analyzing its mechanical behaviour. Through a microscopic characterization of elastic properties of soil grains, the model is generated. The effective percolation threshold obtained is lower than that of the network geometric percolation. The effective mechanical properties predicted are successfully compared to published experimental results.

Strength Degradation Model for Low-Rise Reinforced Concrete Walls Derived from Dynamic and Quasi-Static Tests

Results from a previous experimental program demonstrated that loading rate, strength mechanisms associated with the failure mode, low-cycle fatigue, and cumulative values of displacement, and dissipated energy strongly affect the degradation properties of reinforced concrete (RC) walls for low-rise housing. Thus, data obtained from quasi-static (QS) cyclic tests should not immediately be assumed to represent a conservative lower bound on a specimens capacity. Aimed at numerically correlating results measured during dynamic and QS-cyclic testing, this paper proposes a strength degradation model. The model is readily applicable to seismic design or assessment of performance of existing and new structures. A seismic demand model is also proposed for correlating the intensity and duration of a given earthquake-induced movement to parameters that define the degradation model. Correlations are deemed useful for interpreting the results of QS-cyclic tests and for calibrating hysteretic and behavioral analytical models obtained from QS testing.

Earthquake-Resilient Communities Resilient communities Resilient communities Resilient communities Resilient communities : A Look from Mexico

It is the aim of this chapter to assess the general situation of earthquake resilience in communities in Mexico. This evaluation is performed from a public policy point of view. From the diagnosis presented, challenges and areas of opportunity for implementing programs aimed at reducing risk and attaining more resilience are discussed. It is conjectured that some conclusions and recommendations aimed at achieving resilient communities in the developing countries are also applicable to the developed world.